Optical data storage media typically have scratch resistant protective layers to protect a recording layer from dust and abrasion. In U.S. Pat. No. 4,360,728 to Drexler, a data card has a thin, transparent plastic sheet laminating material or coating covering a strip of optical recording material. In U.S. Pat. No. 4,360,820, Forster et al. disclose a laser recording medium with a protective polymeric coating deposited over all surfaces of the medium.
In U.S. Pat. No. 4,340,655, Hollister discloses a recording medium in which a recording layer is coated with a transparent thermal and mechanical barrier layer of water-soluble polymer. The barrier layer is coated with a water-impermeable spacer layer, so that the medium is resistant to the effects of humidity in the environment. The spacer layer may be coated with an antistatic, abrasion resistant layer.
In U.S. Pat. No. 4,318,970, Kurland et al. teach the use of a glass, optically transparent moisture barrier layer between a hydrophobic substrate, such as polycarbonate material, and a hydrophilic photosensitive layer, such as a gelatin based emulsion. The barrier layer prevents diffusion of water from the substrate to the photosensitive layer.
Recording layers based on metal particles in a gelatin matrix are adversely affected by swelling of the gelatin matrix. Such swelling can change the morphology of recorded spots or pits, degrading the signal from laser reading and introducing errors. Further, the overall reflectivity of the recording surface may be affected, producing inconsistent read signals from pits written at different times. In conditions of elevated temperature (50.degree. C.) and high humidity (90% relative humidity), signals from recorded data spots may be degraded in as short a time as 16 hours. Other recording media, such as dye-polymer and thin film tellurium based media, are also adversely affected by moisture.
Scratch resistant layers and moisture barriers prevent degradation of the recording layers and make optical data storage media durable over long periods of time. However, scratch resistant protection layers do eventually become scratched so that data becomes unreadable. Unless this unreadable data is recorded elsewhere, the data is lost.
Data disks are typically polymethylmethacrylate, polycarbonate or polyethylene terephthalate based with plastic layers at least 40 mils (1 millimeter) thick. Disks are typically used in an office environment, with filtered air and careful handling. By contrast, the total thickness of optical data cards is preferably similar to a credit card thickness of 32 to 34 mils. Small diameter optical disks must also be thin. Cards are also ruggedly treated and subject to dirty environments, being carried about in wallets and pockets. As a result, cards are polycarbonate or polyvinylchloride based and have a base layer only about 15 mils (380 microns) thick. Glass moisture barrier layers as taught by Kurland would be subject to cracking and loss of barrier properties if used in cards.
The object of the invention is to provide an optical data recording and storage medium in which data may be recovered after its protective layer is so scratched that data becomes unreadable.